Mitochondrial Respiration Under Sequential Inhibitors
Isolated bovine heart mitochondria are suspended in a respiration buffer at pH 7.4 and 25 °C with excess ADP and inorganic phosphate. The substrate is 10 mM pyruvate plus 2 mM malate. An oxygen electrode records a state-3 rate of 42 nmol O2 consumed per minute per milligram of protein, and a coupled ATP-synthesis rate of 210 nmol ATP per minute per milligram. The investigator then adds 2 µg/mL oligomycin: O2 consumption falls to 8 nmol/min/mg and ATP synthesis falls to zero. Next, 50 µM 2,4-dinitrophenol (DNP) is added: O2 consumption rises to 55 nmol/min/mg while ATP synthesis remains zero. Finally, 1 mM potassium cyanide is added, and O2 consumption stops. In a parallel chamber, 10 mM succinate replaces pyruvate and malate; the baseline O2 rate is lower, but the inhibitor responses are otherwise similar. The student must decide what each compound reveals about oxidative phosphorylation and predict the succinate P/O ratio.
1. Assign a mechanism to oligomycin, DNP, and cyanide in this experiment, and explain why DNP increases O2 consumption while ATP synthesis stays at zero.
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Oligomycin binds the Fo subunit of ATP synthase and blocks proton flow through the enzyme. Electrons continue briefly, but the proton gradient builds until respiration is suppressed; hence O2 rate falls from 42 to 8 and ATP synthesis is zero. DNP is a proton ionophore: it carries H+ across the inner membrane without passing through ATP synthase, so the gradient is dissipated. The respiratory chain speeds up to 55 nmol O2/min/mg to try to restore the gradient, but the energy is released as heat and ATP synthase has no proton-motive force to use, so ATP synthesis remains zero. Cyanide binds the ferric heme a3 in cytochrome c oxidase (complex IV), blocking reduction of O2 to water; electron transfer stops and O2 consumption goes to zero. The order matters: after cyanide, no substrate can restore O2 consumption because complex IV is the terminal oxidase.
2. Predict the P/O ratio for the succinate experiment and explain whether the oligomycin and DNP responses should differ qualitatively.
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Succinate donates electrons to complex II (succinate dehydrogenase), bypassing complex I. For each pair of electrons, complexes III and IV pump 6 H+ total, whereas NADH oxidation through complexes I, III, and IV pumps 10 H+. With the same ATP synthase, the P/O ratio should be about 1.5 for succinate versus 2.5 for pyruvate/malate. The baseline O2 consumption may be lower because fewer protons are pumped per electron pair, so less respiratory control is available. Oligomycin should still stop ATP synthesis and depress O2 consumption, DNP should still uncouple and stimulate O2 consumption without ATP synthesis, and cyanide should still block complex IV and stop O2 consumption. The qualitative pattern is the same because the inhibitors act downstream of the substrate entry point.
3. A classmate concludes that DNP makes the mitochondria more efficient because O2 consumption rises. Evaluate this claim using the ATP/O data.
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The claim confuses rate with efficiency. Efficiency here is ATP produced per O2 consumed, or ATP/O. Before DNP, 210 nmol ATP/min/mg divided by 42 nmol O2/min/mg gives about 5 ATP per O2, equivalent to a P/O of 2.5, which reflects coupled oxidative phosphorylation. After DNP, ATP synthesis is zero while O2 consumption is 55, so ATP/O is zero. The increased O2 consumption means the chain is working faster to dissipate the proton gradient, not that more ATP is made. DNP is an uncoupler: it increases heat production and can deplete substrates, but it lowers, not raises, the efficiency of ATP capture. A higher O2 rate is therefore evidence against the classmate's conclusion.